When light travels from one medium to another, some portion of the light is reflected from the interface between the two media. For example, typically about 4-5% of the light shining on a clear plastic substrate is reflected at the top surface.
The back lighting for mobile hand held and laptop devices are not effective to provide desired display quality in the presence of the reflection of the external lighting from the top surface and internal interfaces of the display devices, which in turn reduces contrast ratio and can downgrade viewing quality from the interfering image of external objects.
Different approaches have been employed to reduce the reflection of the top surface of display devices. One approach is to use antireflective coatings such as multilayer reflective coatings consisting of transparent thin film structures with alternating layers of contrasting refractive index to reduce reflection. However, it can be difficult to achieve broadband antireflection using the multilayer antireflective coating technology.
Another approach involves using subwavelength surface structure (e.g., subwavelength scale surface gratings) for broadband antireflection, where the phrase subwavelength is used to describe an object or structure having one or more dimensions smaller than the length of the wave with which the object or structure interacts. For suppression of Fresnel reflections from optical surfaces, subwavelength structured features lead to continuous-profile surface-relief grating as an effective medium to minimize reflection for a range of wavelengths greater than the subwavelength structured features on the surface. Methods for creating the subwavelength surface structure (e.g., by lithography) tend to be relatively complicated and expensive.
Reduction of reflection at broad angle is desired in applications related to optical and optoelectronic devices. It is difficult to fabricate deep surface structure providing low reflection (<1.5%) at 60 degree off angle (i.e., 60 degree from normal to the surface) with traditional subwavelength structure surface technology (e.g., by lithography). Micrometer-scale microstructures such as prism has been intensively utilized to reduce reflection at broad angle for solar applications, but this approach tends to result in high haze and is only applicable when light is transporting from low refractive index medium to high refractive index medium.